The corner of the Universe we call home -- our Solar System -- formed 4.6 billion years ago as the newborn Sun spun slowly in the nebulous disk of leftover matter from which it had grown to life.
The slowly-rotating disk of nebula material was extraordinarily broad, stretching well beyond the edges of today's Solar System.
Today we find nine major planets with at least 61 natural satellites or moons, plus thousands of minor planets or asteroids, more than 1,000 observed comets and plenty of meteoroids and other debris leftover from that original stellar formation time. The planet nearest the Sun is Mercury. The farthest is Pluto. Is there another beyond Pluto? Is there a vast cloud of debris out there beyond Pluto, too, from which comets occasionally rip into the inner Solar System?
The original disk. There was a sizeable amount of rocky metallic matter left in the dusty chaos after the Sun formed. And tiny particles of frozen water and frozen carbon dioxide. All were suffused through a vast cloud of hydrogen and helium.
The Sun's nuclear burning threw off vast quantities of heat. Nearby in the boiling soup, metallic grains and chunks of rock formed. Iron and stone floated in a vast cloud of dust. Over time, some collided and stuck together. As that accretion continued over millions of years, large rocky bodies known as planetesimals were formed -- tiny worlds, maybe as large as 60 miles or so in diameter.
After some rocks collided, they broke apart, blasting more chunks through the nebula to crash into more rocks, continuing the process.
After 75 million years or so of such chaos, what we know today as the major and minor inner planets were growing recognizable. The bigger they became, the more their gravity was strong enough to attract smaller chunks, adding to their size.
There were still plenty of planetesimals flying around and, over a few hundred million years, they crashed one after another into various major planets. Some would have been deflected, of course, even out of the Solar System, as the system began to clean itself up.
Inner planets. Mercury, Venus, Earth and Mars are the rocky, metallic, major planets which formed near the Sun. Today we refer to them as terrestrial or planets since they are much like Earth.
The Asteroid Belt, probably composed of smashed planetesimals, is in an orbit farther from the Sun than Mars, but not as far as Jupiter, the first of the outer planets.
Outer planets. The Sun's heat was not as strong farther out in the nebula. Cooler temperatures allowed chunks of ice, floating among the rocks in the hydrogen and helium cloud, to grow.
The four planets formed in these cooler regions -- Jupiter, Saturn, Uranus and Neptune -- were larger worlds of mostly ice and gas. Referred to as Jovian planets after Jupiter, they have very little rock content.
Did the Jovian planets form from accretion or merely coalesce? After all, the Sun coalesced from gases. It's hard to say today, but future explorers may find out.
Was it unusual? It may be that planets don't always form out of a nebula disk of matter around a new star. It seems that many times two or a cluster of stars form as the nebula coalesces. Does that leave additional debris to form planets?
Already unmanned interplanetary spacecraft from Earth have visited the Sun, the Moon, the Asteroid Belt and seven of the eight other planets in our Solar System. Space scientists would like to send a probe to Pluto and astronomers have begun an earnest search outside the Solar System for other stars with planets.
What is it today? The Solar System is a collection of bodies held near each other in space by the gravitational attraction of a star we call the Sun. The bodies orbit the Sun, which in turn is one of a hundred billion stars orbiting the center of our galaxy.
Even today, billions of years after the major planets formed from the disk of matter around the Sun, all of the bodies of the Solar System swim in a thin soup of rocky dust particles. The rocks and dust may be left over from the original formation accretion process, or ejected by comets as they pass through the inner Solar System or they may be debris from collisions between minor planets.
The Sun is the only star we know for a fact to be accompanied by such an extensive swarm of objects, although some astronomers have seen effects that indicate there may be other stars with planets nearby in our own Milky Way galaxy. Could it be that other solar systems exist and have technological civilizations?
Solar System: The Sun Inner System: Mercury Venus Earth Mars Outer System: Jupiter Saturn Uranus Neptune Pluto Other Bodies: Moons Rings Asteroids Comets
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